Catalytic compositions and methods for ethanol oxidation

ABSTRACT

In one aspect, nanoparticles for ethanol oxidation are described herein, which comprise a core including at least one Group IB metal and a shell disposed over the core, wherein the shell comprises islands of alloyed platinum group metals. In another aspect, an electrode is described herein, which in some embodiments, comprises a substrate and electrocatalytic nanoparticles deposited over the substrate. In some embodiments, the electrode described herein further comprises a layer of carbon nanoparticles positioned between the substrate and the electrocatalytic nanoparticles. In yet another aspect, a method of ethanol oxidation is described herein. In some embodiments, such a method comprises (i) providing an electrode comprising a substrate and electrocatalytic nanoparticles deposited over the substrate, (ii) disposing the electrode in an alkaline medium comprising ethanol; and (iii) oxidizing the ethanol with the electrode.

RELATED APPLICATION DATA

The present application claims priority pursuant to 35 U.S.C. § 119(e)to U.S. Provisional Patent Application Ser. No. 62/856,506 which isincorporated herein by reference in its entirety.

STATEMENT OF FEDERALLY SPONSORED RESEARCH

This invention was made with government support under contract no. CBET1703827 awarded by the NSF and contract no. DE-SC0012704 awarded by theDOE. The government has certain rights in the invention.

FIELD

The invention is generally related to devices and methods for ethanoloxidation, and, more specifically, devices and methods for completeethanol oxidation via a C1-12 electron pathway.

BACKGROUND

The hydrogen-rich liquid fuels, such as ethanol, methanol, and ammonia,are attractive alternatives of hydrogen for directly converting chemicalenergy to electricity in fuel cells due to their high energy density,wide availability, and easiness in storage, distribution, and refuel.Among the three promising liquid fuels, ethanol has the highest energydensity. A complete oxidation of ethanol involves transferring 12electrons (12e) per ethanol molecule: CH₃CH₂OH+3H₂O=2CO₂+12H⁺+12e⁻ withCO₂ as the product in acid. However, incomplete ethanol oxidationreaction (EOR) often occurs with 4e transfer producing acetic acid(CH₃COOH) in acid or acetate (CH₃COO⁻) in base. The two pathways aretermed as C1-12e and C2-4e pathways, respectively, referring to thenumber of carbon atoms in the products and the number of electronstransferred per ethanol molecule. In acidic media, a ternary PtRhSnO₂catalyst was the first one showing a 12e EOR to CO₂ at low potentials.The optimal composition, PtRh⅓SnO₂, yields a peak current of 1.5 A mg⁻¹platinum group metal (PGM) and a current of 0.14 A mg⁻¹ PGM after 1 h at0.45 V. In alkaline media, Pd-based catalysts outperform Pt-basedcatalysts with the peak current reaching 40 A mg⁻¹ PGM on Au/Ag/Pd alloyaerogels. However, complete oxidation of ethanol at low potentialsremains a challenge as energy conversion efficiency is low due toincomplete fuel oxidation and high overpotentials. Thus, there exists aneed for improved catalyst performance and understanding of reactionmechanisms.

SUMMARY

In view of these disadvantages, catalysts are described herein thatexhibit not only high peak current during ethanol oxidation, but alsolow onset potential and high selectivity toward the C1-12e pathway.

In one aspect, nanoparticles for ethanol oxidation are described herein.In some embodiments, the nanoparticles comprise a core including atleast one Group IB metal and a shell disposed over the core, wherein theshell comprises islands of alloyed platinum group metals. The alloy ofthe islands can comprise two or more platinum group metals. In someembodiments, the alloyed platinum group metals is platinum-iridiumalloy. The ratio of platinum to iridium in the alloy can be greater than1, in some embodiments. The platinum-iridium alloy, for example, can beof the formula PtIr_(x), wherein x ranges from 0.1-0.95 to establish Iras a fractional value of the Pt content in the alloy. In someembodiments, x ranges from 0.5-0.92 or 0.6-0.7.

The nanoparticles, in some embodiments, have an average diameter of 3-8nm. Additionally, in some instances, a molar ratio of the alloyedplatinum group metals in the shell to the Group IB metal of the coreranges from 0.1 to 0.2. In some embodiments, the core comprises gold orsilver. Moreover, the core can be an alloy of Group IB metals, in someembodiments. For example, the core can be an alloy of silver and gold.

In some embodiments, the Group IB metal of the core induces latticeexpansion in the islands of the alloyed platinum group metals. Suchlattice expansion can induce tensile stress in the islands.Additionally, the Group IB metal of the core, in some instances, has asingle crystal structure. The core, for example, can be formed of gold,silver or alloys thereof. It is contemplated that the core can have anydesired shape and/or morphology. The core can be spherical, polygonal,elliptical, star-shaped, dendritic, platelet shape or cubed. Shape ofthe core can be selected according to several considerations, includingdesired compressive stress or tensile stress condition of the islands.

The islands of alloyed platinum group metals can exhibit monolayerthickness, in some embodiments. For example, the islands can be formedof a single atomic layer. Additionally, in some embodiments, the islandsof alloyed platinum group metals form an interface with the core.Alternatively, one or more metal or alloy layers be reside between thecore and islands. In some cases, the islands of alloyed platinum groupmetals comprise platinum-iridium alloy.

In another aspect, an electrode is described herein, which comprises asubstrate and electrocatalytic nanoparticles deposited over thesubstrate. Any nanoparticle described herein can be used in theelectrode construction. In some cases, the electrocatalyticnanoparticles comprise a core-shell architecture, wherein the corecomprises at least one Group IB metal, and the shell comprises islandsof alloyed platinum group metals. The electrocatalytic nanoparticles, insome instances, have an average size of 3-8 nm. The islands of alloyedplatinum group metals can exhibit monolayer thickness and/or comprise aplatinum-iridium alloy. Moreover, the Group IB metal of the core, insome embodiments, induces lattice expansion in the islands of alloyedplatinum group metals. In some embodiments, the electrode describedherein further comprises a layer of carbon nanoparticles positionedbetween the substrate and the electrocatalytic nanoparticles.

In some embodiments, the electrocatalytic nanoparticles of electrodesdescribed herein can provide one or more features or characteristics ofthe electrode. For example, in some cases, the electrocatalyticnanoparticles can provide peak current of at least 50 A/mg of platinumgroup metal during ethanol oxidation in alkaline media. Theelectrocatalytic nanoparticles, in some cases, can also provide an onsetpotential of 0.4 V to 0.5 V for ethanol oxidation. Furthermore, theelectrocatalytic nanoparticles can be selective to a C1-12 electronpathway for ethanol oxidation, and the C1-12 electron pathway canaccount for greater than 50 percent of current generated during ethanoloxidation.

In yet another aspect, a method of ethanol oxidation is describedherein. In some embodiments, such a method comprises (i) providing anelectrode comprising a substrate and electrocatalytic nanoparticlesdeposited over the substrate, (ii) disposing the electrode in analkaline medium comprising ethanol; and (iii) oxidizing the ethanol withthe electrode. Any nanoparticle and/or electrode described herein can beused in any one or more methods of ethanol oxidation. In someembodiments, the electrocatalytic nanoparticles comprise a core-shellarchitecture, wherein the core comprises at least one Group IB metal,and the shell comprises islands of alloyed platinum group metals.

These and other embodiments are described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an X-ray diffraction profile of Au@PtIr/C comparedto PtIr/C showing Au-induced lattice expansion.

FIG. 1B show a TEM image of well dispersed Au@PtIr particles on carbonsupport with narrow particles size distribution. The insert shows atomicspacing of 2.35 Å consistent with Au{111} facets.

FIG. 1C shows a HAADF-STEM image with line profiles of EDS elementaldistributions of Au, Pt, and Ir across a single particle.

FIG. 1D-G shows two-dimensional mapping of Au-Lα, Pt-Lα, and Ir-Lα EDSintensities.

FIG. 2A illustrates the PGM mass activities of three Au core catalysts:Au@PtIr/C Au@Pt/C, and Au@Ir/C.

FIG. 2B illustrates metal mass activities of the three core-shellcatalysts Au@PtIr/C Au@Pt/C, and Au@Ir/C) and a PtIr (1:1 molar ratio)alloy catalyst.

FIG. 2C illustrates PGM mass activities of Au@PtIr without and withfunctional carbon nanotube (f-CNT) on gas diffusion electrodes.

FIG. 2D illustrates the durability measured by chronoamperometry at 0.45V versus RHE.

FIG. 3A-3B illustrates the in-situ IRRAS spectra for the products (FIG.3A) and adsorbed intermediates (FIG. 3B) of ethanol oxidation in base byAu@PtIr/C.

FIG. 3C-3D illustrates the in-situ IRRAS spectra for the products (FIG.3C) and adsorbed intermediates (FIG. 3D) of ethanol oxidation in base byPtIr/C.

FIG. 3E-3F illustrates the in-situ IRRAS spectra for the products (FIG.3E) and adsorbed intermediates (FIG. 3F) of ethanol oxidation in base byAu@P/Ct.

FIG. 4A illustrates the integrated absorbances of EOR products (Leftaxis) and molar ratio of carbonate to acetate (Right axis) for Au@Pt/Cand PtIr/C.

FIG. 4B illustrates the integrated absorbances of EOR products (Leftaxis) and molar ratio of carbonate to acetate (Right axis) forAu@PtIr/C.

FIG. 4C is a schematic of the proposed direct C1-12e, C2-4e, andindirect C1-12e pathways of ethanol oxidation.

FIG. 5A illustrates voltammetry curves normalized to metal surface areascalculated from average metal densities and diameter of particles, usingmass-specific surface area=6000/diameter/density.

FIG. 5B illustrates area-specific peak current versus peak potential(bars) for EOR in 1 M KOH and 1 M ethanol on Au@Ir/C, PtIr/C, Au@Pt/C,and Au@PtIr/C catalysts. The curved line is an EOR polarization curvefor Au@PtIr/C. Values for unary Ir(111), Pt(111), and Au(111) arereferred to the results on (111) surfaces reported in literature. Theinsert is a schematic illustration of atoms at edges of a 2.3-nmsphere-like nanoparticle.

FIG. 5C shows schematic illustrations of atomic models for surfacelayers with Pt, Ir, and a core of Au. Lateral expansion of Pt spacing isillustrated by black arrows and atomic steps are shown by gray lines.

FIG. 6A illustrates EOR performance of Au@PtIr_(x) catalysts in 1 M KOHand 1 M ethanol at 20 mV/s measured from 0.05-1.0 V vs RHE.

FIG. 6B is a magnified view of the boxed area of FIG. 6A.

FIG. 7 illustrates PGM mass activity of the Au@PtIr_(x) electrocatalystsat 0.45 V, 0.6 V and peak potential.

DETAILED DESCRIPTION

Embodiments described herein can be understood more readily by referenceto the following detailed description and examples. Methods, devices,and features described herein, however, are not limited to the specificembodiments presented in the detailed description and examples. Itshould be recognized that these embodiments are merely illustrative ofthe principles of the present disclosure. Numerous modifications andadaptations will be readily apparent to those of skill in the artwithout departing from the spirit and scope of the disclosure.

In addition, all ranges disclosed herein are to be understood toencompass any and all subranges subsumed therein. For example, a statedrange of “1.0 to 10.0” should be considered to include any and allsubranges beginning with a minimum value of 1.0 and ending with amaximum value of 10.0 or less, e.g. 1.0 to 5.3, or 4.7 to 10.0, or 3.6to 7.9.

All ranges disclosed herein are also to be considered to include the endpoints of the range, unless expressly stated otherwise. For example, arange of “between 5 and 10,” “from 5 to 10,” or “5-10” should generallybe considered to include the endpoints 5 and 10.

Further, when the phrase “up to” is used in connection with an amount orquantity, it is to be understood that the amount is at least adetectable amount or quantity. For example a material present in anamount “up to” a specified amount can be present from a detectableamount and up to and including the specified amount.

EXAMPLES

The following nanoparticle compositions were fabricated andcharacterized as examples of the compositions and methods describedherein. Accordingly, the present disclosure is not limited to thespecific compositions and methods described in the examples.

PtIr on carbon supported Au (Au/C) nanoparticles were synthesized bysimultaneously reducing Pt and Ir precursors onto Au nanoparticles in anaqueous solution at a temperature near to boiling. This method waschosen to promote the formation of a well-mixed atomic layer of Pt andIr on Au nanoparticles. The molar ratio of metals was 10:1:1 forAu:Pt:Ir based on an estimated one monolayer coverage of Pt and Ir on5-nm Au particles. As shown in FIG. 1a , the resulting Au@PtIrcore-shell nanoparticles exhibit X-ray diffraction peaks at thepositions close to those for face-centered-cubic Au, indicating that thePt and Ir atoms are in registry with the underlying Au lattice(a_(Au)=0.408 nm). The tensile strains calculated from the peakpositions are 3.8% relative to Pt (a_(Pt)=0.392 nm) and 6.1% relative toIr (a_(Ir)=0.386 nm). These values are close to the lattice mismatchesof 4.1% and 6.3%, respectively. In contrast, the lattice spacing of PtIralloy particles is between those of Pt and Ir, resulting in tensilestrained Ir but compressively strained Pt.

The transmission electron microscopy (TEM) images shown in FIG. 1bverifies a narrow particle size distribution of ±1.8 nm and a latticespacing of Au {111} facets. Note that the surface to core atomic ratiofor 5.8±1.8 nm particles calculated using a cuboctahedra model is 0.19and the molar ratio of 10:1:0.7 for Au:Pt:Ir determined by x-rayabsorption spectroscopy yields a 0.17 (Pt+Ir) to Au ratio. Thus, thePtIr shell is essentially in the form of monoatomic islands. FIGS. 1c-gshow the elemental distribution of a representative Au@PtIr particle.The images of energy dispersive spectroscopy mapping show that signalsfor Pt (blue) and Ir (green) spread to a slightly larger circle thanthat for Au (pink) in the two-dimensional projections, suggesting auniformly mixed PtIr atomic layer on the Au particle.

FIG. 2a shows an EOR peak current of 58 A mg⁻¹ PGM obtained using theAu@PtIr/C catalyst in 1 M ethanol and 1 M KOH solution, which is 8 and38 times of the peak currents on Au@Pt/C and Au@Ir/C, respectively. On(111) single crystal surfaces, the EOR peak current on Pt is aboutfivefold of that on Ir, while the EOR onset potential is lower on Irthan on Pt by about 0.25 V.16 This trend persists for the Au@Pt/C andAu@Ir/C catalysts as shown by the blue and green curves in FIG. 2a . Itis remarkable that the combination of Pt and Ir on Au results in aneightfold increase in peak current and a 0.25 V reduction of onsetpotential compared to Au@Pt/C. The EOR currents normalized to the massof all metals are shown in FIG. 2b . At 0.6 V, Au@PtIr/C exhibited 0.52A mg⁻¹, which is 4.3 times of that for a commercial PtIr/C catalyst (1:1Pt:Ir alloy). These measurements were carried out using gas diffusionelectrodes, focusing on the EOR performance below 700 mV where theactivity matters the most for fuel cell applications.

Further enhancement of EOR current at low potentials was obtained byadding acid-treated functional carbon nanotube (f-CNT) on gas diffusionlayer before loading Au@PtIr/C catalyst. FIG. 2c shows that the presenceof f-CNT lowers the onset potential for EOR and increases the EORcurrents at potentials below 0.55 V. At 0.45 V, the EOR current doublesthat in the absence of f-CNT. The same activity increase was found bythe chronoamperometry measurements at 0.45 V. As shown in FIG. 2d , thecurrents at 15 minutes on the Au@PtIr catalysts with and without f-CNTare 0.38 and 0.18 A mg⁻¹, respectively, which are ten times of those forPd/C (˜0.03 A mg⁻¹) and Pt/C (˜0.01 A mg⁻¹).17 With the f-CNT, thecurrents on Au@PtIr at 30 and 60 minutes are 0.34 and 0.30 A mg⁻¹,respectively, which are about twice of those (0.17 and 0.14 A mg⁻¹) onthe best catalyst, PtRh1/3/SnO₂, for EOR in acid.

Table 1 summaries the PGM activities of the Au@PtIr core-shell catalystand the most active EOR catalysts of various types previously reported.The peak current in forward potential sweep given in the fifth column iscommonly used for comparing catalyst performance. In most cases, thepeak potentials are above 0.7 V, out of the overpotential range for ananode in fuel cells. Thus, activities at 0.45 and 0.6 V are listed inthe third and fourth columns, respectively. The second column providesthe EOR activity after 30 min at 0.45 V. As the list shows, Pd-based andPdPt-based catalysts performed better than Pt-based catalysts prior tothis work for EOR in alkaline solutions. Differing from EOR in acid,where Rh and Sn oxide greatly promoted the activity on Pt, Au isinvolved in the most active Pt-based (this work) and Pd-based catalystsfor EOR in alkaline solutions. Atomic structure at the catalyst surfaceplays a very important role in determining the EOR activity; one exampleis the AuPtIr alloy particles being orders of magnitude less active thanthe Au@PtIr core-shell catalysts.

To gain insights in how metal components and atomic structure atnanoparticle surfaces affect EOR selectivity toward to C1-12e pathway,onset potential, and peak current, in-situ IRRAS studies of EOR wereperformed in 1 M KOH on the ternary Au@PtIr core-shell catalyst and itstwo binary counterparts: PtIr alloy and Au@Pt core-shell catalysts. ForEOR in acid, the appearance of the 2343 cm⁻¹ CO₂ band indicates theformation of C1-12e product. In alkaline solutions, CO₂ reacts with OH—to form carbonate and the CO₂ band appears only when pH is ≤13. FIG. 3ashows the spectra acquired during a positive potential sweep forAu@PtIr/C, in which the 2343 cm⁻¹ CO₂ band emerges at potentials above0.8 V. This feature indicates that the C1-12e pathway is active on theternary catalyst and that the pH in the thin layer solution betweenelectrode and optical window can be lowered significantly by EOR duringthe time for a potential scan at 1 mV s−1 from 0.05 V to 0.8 V.

To evaluate selectivity between C1-12e and C2-4e pathways at lowpotentials, the characteristic bands for carbonate and acetate wereanalyzed. In transmission spectra, a single band at 1390 cm⁻¹ wasobserved for Na₂CO₃ while CH₃COONa exhibited two bands at 1550 and 1415cm⁻¹ plus a narrow and weak band at 1348 cm⁻¹. As illustrated in FIG. 3b, fitting the top spectrum (908 mV) with three Lorentzian peaksdeconvolutes the overlapped bands in the region from 1430 to 1260 cm⁻¹.The areas under the 1408 and 1394 peaks are taken as the integratedabsorbance for CH₃COO— and HCO₃—/CO₃ ²⁻, respectively. The third fittedpeak at 1346 cm⁻¹ is broad, differing from the narrow 1348 cm⁻¹ bandfrom acetate (ignored in fitting), and is likely due to the formation ofsmall amount of HCOO⁻. This is supported by the assignment of 1340 cm⁻¹to adsorbed formate on Pt(111).26 Other characteristic bands include asmall band at 923 cm⁻¹ for acetaldehyde, a bipolar band at 1044 cm⁻¹ dueto adsorbed ethanol, and a band that shifts from 1956 to 1985 cm⁻¹ withincreasing potential (FIG. 3c ), which is assigned to linearly adsorbedCO.

Integrated absorbances for EOR products are plotted as a function ofpotential for Au@Pt and PtIr in FIG. 4a and for Au@PtIr in FIG. 4b . Therising absorbances for acetate (circles) and carbonate (triangles)reflect the concentration increases for C2-4e and C1-12e products,respectively. After reaching maximal values around 0.7 V, theseabsorbances slightly decrease due to depletion of reactants in the thinsolution layer and products diffusing away. To determine the selectivitytoward C1-12e pathway at potentials below 0.7 V, the ratio for chargesgenerated from forming carbonate and acetate were calculated from theratio of absorbances using C_(Carbonate)/C_(Acetate)=6A1394/4A1408/2.2,in which 6 and 4 are the electron transfer numbers per carbonate andacetate, respectively; 2.2 is the absorbance coefficient ratiodetermined by measuring absorbance ratio for a solution containing equalmolar amounts of carbonate and acetate. As shown in FIG. 4b (rightaxis), the C_(Carbonate)/C_(Acetate) ratio is nearly a constant of 1.3between 0.3 and 0.7 V for Au@PtIr/C. Since the band at 1348 cm⁻¹ forHCOO⁻ (a C1-10e product) and the 923 cm⁻¹ band CH₃CHO (a C2-2e product)are weak, the percentage of current generated by C1 pathway can beestimated by counting the charges associated with the two majorproducts, i.e.,C_(Carbonate)/(C_(Carbonate)+C_(Acetate))=1.3/(1.3+1)=0.57. Thus, about57% EOR current is generated via a C1-12e pathway on Au@PtIr/C.

The fact that Au@Pt/C also has a high C_(Carbonate)/C_(Acetate) ratio of˜1.2 but there is no CO₂ band (FIG. 3e ) can be explained by the highonset potential (˜0.5 V) and the low rate in producing carbonate andacetate (FIG. 4a ), which means insufficient OH consumption to lower thelocal pH for CO2 band to emerge. On the other hand, there is nomeasurable absorbance for carbonate in the spectra obtained with PtIr/Cas one can see that the peaks centered at 1411 cm⁻¹ in FIG. 3d aresymmetric without a shoulder at 1394 cm⁻¹. Yet, a weak CO₂ band emergesin the spectra above 0.8 V for PtIr/C (FIG. 3e ). These distinctbehaviors can be rationalized by two types of C1-12e pathways asillustrated in FIG. 4 c.

The direct C1 pathway has the C—C bond split occurring viapotential-independent ethanol dissociation, followed by dehydrogenationand oxidation steps without strongly adsorbed intermediates. It competeswith non-dissociative adsorption of ethanol, which can be dehydrogenatedwithout or with C—C bond split. The latter is termed as indirect C1-12epathway for the C—C bond splits after partial dehydrogenation and hasadsorbed CO as a major intermediate. The high and constantC_(Carbonate)/C_(Acetate) ratio at potentials from onset to peak, aswell as the absence of *CO and other strongly adsorbed intermediates inthe spectra for EOR on Au@PtIr and Au@Pt catalysts, indicate that thedirect C1-12e pathway is activated on these core-shell catalysts. Incontrast, PtIr alloy nanoparticles provide an example for the indirectC1-12e pathway. The appearance and growth of the 1970 cm⁻¹ band foradsorbed *CO (cross signs in FIG. 4a ) concurs with the lack ofcarbonate, indicating that oxidation of strongly adsorbed CO is therate-limiting step for C1-12e pathway below 0.8 V on PtIr/C.

The EOR pathway diagram in FIG. 4c indicates that formation of stronglyadsorbed *CO is most likely when C—C bond cleavage occurs at partiallydehydrogenated intermediates, while incompletely dehydrogenated C1intermediates, CH₂O and CHO, are not site-blocking adsorbates.Therefore, high coverage of adsorbed ethanol means low rate of ethanoldissociation and low selectivity toward direct C1-12e pathway. Therationale is supported by higher coverage of non-dissociated ethanol onPtIr/C than on Au@PtIr/C and Au@Pt/C, especially at low potentials. Theevidence is the larger amplitude of the bipolar band near 1046 cm⁻¹ inFIG. 3d than in FIGS. 3b and 3f . A bipolar band arises from the peakposition difference between the spectrum obtained at a high potentialand the reference spectrum. The downward lobe envelop shows decreasingcoverage of adsorbed ethanol with increasing potential from thereference potential of 0.05 V and a small upward lobe emerges due to apotential-induced shift of the band center. For the spectra at thehighest potential in FIGS. 3b and 3d , the bipolar band can bereplicated by the orange curve obtained by subtracting the black curve,A(_(50mV)) from the green curve A(_(E)). The integrated absorbances(given in the inserts) are higher at both low and high potentials forPtIr/C than those for Au@PtIr/C.

The competition between C2 and indirect C1 pathways is largelydetermined by the barriers for oxidation of acetaldehyde and C—C bondcleavage of partially dehydrogenated EOR intermediates. Densityfunctional theory (DFT) calculations have found that among the (100)surfaces of Pd, Pt, and Ir, Ir(100) has the highest barrier for formingCH₃COOH from CH₃CO+OH and the lowest barrier for C—C bond cleavage inCHCO to form CH+CO species. This theoretical result is consistent withthe present disclosure of stronger CO adsorption band at ˜1970 cm⁻¹ forEOR on PtIr alloy nanoparticles than on Pt nanoparticles (barely visiblein FIG. 6A of a previous publication).

FIGS. 5a and 5b summarize the voltammetry and EOR peak currentsnormalized to the calculated metal surface area. Hydrogen adsorption issignificantly stronger on PtIr/C than the core-shell catalysts with Auas the cores. The bars in FIG. 5b show EOR peak currents versus peakpotential. Results for EOR on (111) single crystals of Ir, Pt and Au areincluded to show the property of metals without nanostructure-inducedeffect. Nanometer-sized particles are rich of low-coordination sites atedges, which can be highly active for many reactions. For example, a 4.5nm cuboctahedron Au particle has about 10 atoms at each of 24 edges,which account for ˜20% surface atoms. While there is no EOR currentbelow 0.9 V on Au(111) single crystal surface, EOR current was observedon Au nanoparticles at potentials comparable to that of Ptnanoparticles. For EOR in acid, the CO₂ band at 2343 cm⁻¹ was observedfor Pt monolayer and sub-monolayer on Au nanoparticles, but not for Ptmonolayer on Au(111) single crystals. Thus, ethanol dissociation islikely facile at the monoatomic steps of Pt or PtIr islands, especiallythose near the edges of Au nanoparticles.

Gold cores induce a lateral lattice expansion of Pt and Ir monolayers.The tensile strain has been shown highly effective in enhancing EORcurrent in acid, especially at high potentials. Relative to Pt(111), Ptmonolayer on Au(111) exhibits a fourfold peak current, while Ptmonolayers on Pd(111), Ir(111), Rh(111), and Ru(0001) are compressed andthus result in lower peak currents than that on Pt(111). The effect onpeak current is even higher for methanol oxidation reaction as asevenfold enhancement is seen on Pt monolayer/Au(111) relative toPt(111). Thus, oxidation of either C1 or C2 intermediates are benefitedfrom the Au-induced tensile strain that enhances water and OH adsorptionin acid and base, respectively. In this study, the six orders ofmagnitude higher EOR activity on Au@PtIr core-shell catalysts than onAuPtIr alloy catalysts demonstrates the role of both the atomic stepsfor promoting direct C1 pathway and the tensile strain for improvingoxidation kinetics. This discovery can guide catalyst design andsynthesis optimization.

Iridium, on the other hand, acts as a promotor for dehydrogenation,which lowers the onset potential. Ammonia oxidation reaction,2NH₃+6OH⁻=N₂+6H₂O+6e⁻, involves dehydrogenation but not oxidationprocess. DFT calculations have found Ir as the best metal fordehydrogenation of ammonia, and experimental studies have shown theimportance of Ir for lowering the onset potential for ammonia oxidation.For EOR in base, the onset potential for producing carbonate/acetate isin the order of Au@PtIr<PtIr<Au@Pt (FIGS. 4a and 4b ), supportingdehydrogenation as the rate limiting step at low potentials for EOR andIr being a dehydrogenation promotor. The eightfold activity enhancementby adding Ir to Au@Pt demonstrates the impact of the synergy generatedfrom monatomic steps for ethanol dissociation, Ir for dehydrogenation,and tensile strain for oxidation.

In conclusion, the ternary core-shell catalyst, Au@PtIr/C, exhibitsextraordinary EOR performance in alkaline solution—a high peak currentof 58 A mg⁻¹ (PGM) and 8.3 A g⁻¹ (all metals), a low onset potential of0.3 V, and a high percentage C1-12e current of 57%. All three measurestogether significantly improved the efficiency of ethanolelectrooxidation. This is achieved by activating a direct C1-12epathway, in which the C—C bond splits via ethanol dissociation andsite-blocking CO intermediate is circumvented. In addition, cleavage ofthe C—C bond prior to electrochemical dehydrogenation of ethanolprovides C1 fragments at low potentials. These intermediates are lessstable than ethanol, and thus, offer opportunities for reducing the EORonset potential. Adding functional carbon nanotubes lowered the EORonset potential to 0.2 V, which bodes well for future study along thisdirection. Comparison of peak currents normalized to metal surface areasfor the ternary Au@PtIr core-shell catalyst and its binary and unarycounterparts show remarkable sensitivity of EOR kinetics onthree-dimensional atomic structure at metal surfaces. Monoatomic stepsof Pt or PtIr islands on Au cores activate the direct C1-12e pathway,while smooth surface of PtIr alloy particles results in the indirectC1-12e pathway that has the oxidation of strongly adsorbed *CO as therate-limiting step. Ir and Au are inactive for EOR due to too strong andtoo weak adsorption of reaction intermediates, respectively. Placing Irat the surface and Au in the core allows them to play complementaryroles in promoting dehydrogenation and oxidation of both C1 and C2intermediates. These results and new insights are encouraging foradvancing direct liquid fuel cells and point directions for futurestudies of EOR catalysts and other electrocatalytic reactions.

In a separate study, the shell composition ratio in theAu(core)-PtIr(shell) nanoparticles for EOR was varied. It was found thatshell composition ratios of Pt:Ir>1 performed better than shellcomposition ratios of Pt:Ir<1. This result suggests that Pt is the majorplayer responsible for the EOR electrocatalysis, while Ir is thepromoter for dehydrogenation to lower the overpotential of the reaction.Further refining the ratio between PtIr0.59 to PtIr0.95 indicates thatthe ratio of PtIr0.65 exhibited the highest performance. EOR performanceand mass activity of the Au core-PtIr_(x) shell electrocatalysts areprovided in FIGS. 6A and 6B, respectively. A summary is provided inTable I.

TABLE I Summary of PGM mass activity (A mg_(PGM) ⁻¹) of the Au@PtIr_(x)electrocatalysts at 0.45 V, 0.6 V and peak potential 0.45 V 0.6 V peakAu@Pt 0.82 5.66 17.70 Au@PtIr0.59 0.84 6.80 33.12 Au@PtIr0.65 1.01 11.8653.19 Au@PtIr0.81 0.86 8.35 39.67 Au@PtIr0.91 0.50 7.94 37.61

Methods

Chemicals and Materials.

All metal precursors, HAuCl₃.H₂O, IrC₃.H₂O, and K₂PtCl₄ purchased fromSigma-Aldrich were used without further purifications. Vulcan 72R wasused as the carbon support.

Synthesis of 20 wt. % Au/C.

The 20 wt. % Au/C was synthesized by the reduction of Au precursor usingsodium borohydride, followed by the loading to the carbon support. In atypical synthesis, 2 mL of 1 wt. % HAuCl₃.H₂O aqueous solution and 2 mLof 1 wt. % trisodium citrate aqueous solution were added to 200 mL ofwater in a round bottom flask. One minute after the solution mixing, 2mL of 0.075 wt. % sodium borohydride in 1 wt. % trisodium citratesolution was added to the reaction solution under vigorous stirring.After another 5 min, 40 mg of carbon black were added to the solution.The reaction proceeded under vigorous stirring for additional 2 h. Theproduct was purified by water twice and ethanol once, collected byvacuum filtration, and dried under vacuum overnight for further use.

Synthesis of Ir on 20 wt. % Au/C.

The Ir on 20 wt. % Au/C was synthesized by the reduction of Ir precursorusing ascorbic acid at in the presence of 20 wt. % Au/C. In a typicalsynthesis, 60 mg of ascorbic acid and 50 mg of Au/C (20 wt. %) was addedto 10 mL of water in a 100 mL round bottom flask. After the mixture inwater was heated boiled, 6 mL of 0.5 mg/mL IrCl₃.H₂O aqueous solutionwas added to the mixture at a rate a rate of 2.0 mL/h. After theaddition of Ir precursor, the reaction proceeded under vigorous stirringfor additional 2 h to overnight. The product was purified by water twiceand ethanol once, collected by vacuum filtration, and dried under vacuumovernight for further use.

Synthesis of Pt and Ir on 20 wt. % Au/C in Water.

The Pt—Ir shell on 20 wt. % Au/C core was synthesized by the reductionof Pt and Ir precursors using ascorbic acid at in the presence of 20 wt.% Au/C. In a typical synthesis, 60 mg of ascorbic acid and 50 mg of Au/Cwas added to 10 mL of water in a 100 mL round bottom flask. After themixture in water was heated to boil, 3 mL of 0.5 mg/mL IrCl₃.H₂O and 3mL of 0.7 mg/mL K₂PtCl₄ aqueous solutions were simultaneously added tothe mixture at a rate of 2.0 mL/h. After the addition of Pt and Irprecursors, the reaction proceeded under vigorous stirring foradditional 2 h to overnight. The product was purified by water twice andethanol once, collected by vacuum filtration, and dried under vacuum.The molar ratio determined by XAS for Au:Pt:Ir is 10:1:0.7 (within <5%error), close to that in the precursors.

Characterization.

X-ray diffraction (XRD) profiles of catalyst samples were collectedusing Cu Kα radiation (λ=1.5418 Å). Transmission electron microscopy(TEM) images were obtained using a JEOL JEM-ARM200CF operated at 200 kV.

Catalyst Ink and Electrode Preparation.

Catalyst inks were made with a mixed solvent of deionized water,ethanol, and isopropanol (volume ratio 1:1:2) containing Nafion with 0.1to 0.3 weight ratio to the carbon weight in the catalyst. Aftersonicating in ice water bath for more than 20 min, 5 to 15 μL ofcatalyst ink was dropped onto glassy carbon electrode to have a catalystloading of 0.1 mg cm⁻². The total metal loadings were about 0.02 mgcm⁻². For gas diffusion electrode samples, a calculated amount of inkwas placed over 1 cm² area at one end of a 1.4 cm wide and 3 cm long gasdiffusion layer (Sigracet 25 BC) to get total metal loading about 0.1 mgcm⁻².

Electrochemical Measurements.

A Voltalab PGZ 402 potentiostat was used for electrochemical measurementwith a Hg/HgO electrode as the reference electrode and a Pt-flag as thecounter electrode in 1 M KOH solution. The zero potential versusreversible hydrogen electrode (RHE) in 1 M KOH was determined by theopen circuit potential on Pt in hydrogen saturated solution. The iR-freepotential was obtained by subtracting the product of measured currentsand the high-frequency resistance determined from electrochemicalimpedance spectra acquired at 500 mV versus RHE.

In-Situ Infrared Reflection Absorption Spectroscopy (IRRAS).

In-situ IRRAS measurements were carried out with a Nicolet iS50 FT-IRspectrometer equipped with an A-type MCT detector cooled with liquidnitrogen. The working electrodes were made via casting appropriateamounts of catalyst inks on a gold disk electrode. The loadings ofcarbon supported catalysts are ˜0.05 mg cm⁻² total metals. A Hg/HgOelectrode was used as the reference electrode and Pt was the countelectrode. During in situ IRRAS measurements, the working electrode waspressed against a ZnSe hemisphere. The spectral resolution was set at 4cm⁻¹ and the reference reflectivity (R_(ref)) was taken at 50 mV vs.RHE. Absorbance spectra, −log(R/R_(ref)), are presented. Each ofpotential-dependent spectra was acquired by integrating 288interferograms collected in 50 s. The potential for each spectrum is theaverage potential between that at the beginning and end of datacollection while the potential was continuously increased at 1 mV s⁻¹.

Many modifications and other embodiments of the subject matter will cometo mind to one skilled in the art to which the subject matter pertainshaving the benefits of the teachings presented in the foregoingdescriptions and the associated drawings. For example, although specificconfigurations of nanoparticles are described above and depicted in thefigures, numerous other nanoparticles selective to a C1-12 electronpathway for ethanol oxidation and/or configured to provide a peakcurrent of at least 50 A/mg of platinum group metal during ethanoloxidation in alkaline media may benefit from embodiments of the subjectmatter described herein. Therefore, it is to be understood that thesubject matter is not to be limited to the specific embodimentsdisclosed and that modifications and other embodiments are intended tobe included within the scope of the appended claims. Although specifictetras are employed herein, they are used in a generic and descriptivesense only and not for purposes of limitation.

Various implementations of devices and methods have been described, andexemplary embodiments are described below in fulfillment of variousobjectives of this disclosure. It should be recognized that theseimplementations are merely illustrative of the principles of thisdisclosure. Numerous modifications and adaptations thereof will bereadily apparent to those skilled in the art without departing from thespirit and scope of this disclosure. For example, individual steps ofmethods described herein can be carried out in any manner notinconsistent with the objectives of this disclosure, and variousconfigurations or adaptations of devices described herein may be used.

1. A nanoparticle comprising: a core including at least one Group IBmetal; and a shell disposed over the core, the shell comprising islandsof alloyed platinum group metals.
 2. The nanoparticle composition ofclaim 1, wherein the islands of alloyed platinum group metals exhibitmonolayer thickness.
 3. The nanoparticle composition of claim 1, whereinthe islands comprising platinum-iridium alloy.
 4. The nanoparticlecomposition of claim 3, wherein a ratio of platinum to iridium in thealloy is greater than
 1. 5. The nanoparticle composition of claim 3,wherein the platinum-iridium alloy is of the formula PtIt_(x) with xbeing 0.6-0.7.
 6. The nanoparticle composition of claim 1, wherein thecore comprises gold, silver or an alloy of gold and silver.
 7. Thenanoparticle composition of claim 1, having a diameter of 3-8 nm.
 8. Thenanoparticle of claim 1, wherein the Group IB metal of the core induceslattice expansion in the islands of alloyed platinum group metals. 9.The nanoparticle composition of claim 8, wherein the islands of alloyedplatinum group metals exhibit tensile stress.
 10. The nanoparticlecomposition of claim 1, wherein the islands of alloyed platinum groupmetals form an interface with the core.
 11. The nanoparticle of claim 1,wherein at least one Group IB metal of the core has a single crystalstructure.
 12. The nanoparticle of claim 1, wherein a molar ratio of thealloyed platinum group metals in the shell to the Group IB metal of thecore ranges from 0.1 to 0.2.
 13. The nanoparticle of claim 1, whereinthe core is spherical or elliptical.
 14. The nanoparticle of claim 1,wherein the core is polygonal.
 15. An electrode comprising: a substrate;and electrocatalytic nanoparticles deposited over the substrate, theelectrocatalytic nanoparticles comprising a core-shell architecture,wherein the core comprises at least one Group IB metal, and the shellcomprises islands of alloyed platinum group metals.
 16. The electrode ofclaim 15 further comprising a layer of carbon nanoparticles positionedbetween the substrate and the electrocatalytic nanoparticles.
 17. Theelectrode of claim 15, wherein the electrocatalytic nanoparticles havean average size of 3-8 nm.
 18. The electrode of claim 15, wherein theelectrocatalytic nanoparticles provide a peak current of at least 50A/mg of platinum group metal during ethanol oxidation in alkaline media.19. The electrode of claim 18, wherein the electrocatalyticnanoparticles provide a peak current of at least 50-60 A/mg of platinumgroup metal during the ethanol oxidation.
 20. The electrode of claim 15,wherein the electrocatalytic nanoparticles are selective to a C1-12electron pathway for ethanol oxidation.
 21. The electrode of claim 15,wherein the electrocatalytic nanoparticles provide an onset potential of0.4 V to 0.5 V for ethanol oxidation.
 22. The electrode of claim 20,wherein the C1-12 electron pathway accounts for greater than 50 percentof current generated during ethanol oxidation.
 23. The electrode ofclaim 15, wherein the islands of alloyed platinum group metals exhibitmonolayer thickness.
 24. The electrode of claim 15, wherein the islandscomprise platinum-iridium alloy.
 25. The electrode of claim 15, whereinthe Group IB metal of the core induces lattice expansion in the islandsof alloyed platinum group metals.
 26. A method of ethanol oxidationcomprising: providing an electrode comprising a substrate andelectrocatalytic nanoparticles deposited over the substrate, theelectrocatalytic nanoparticles comprising a core-shell architecture,wherein the core comprises at least one Group IB metal, and the shellcomprises islands of alloyed platinum group metals; disposing theelectrode in an alkaline medium comprising ethanol; and oxidizing theethanol with the electrode.